Apologies in advance if this is a completely uninformed question...why not just interface directly with Google Calender, Google contacts, etc directly through the iPhone Safari browser? The only answer I can think of is that you are limited to working when you have cellular or WiFi service. Any other reasons? Thanks.
Google mail and calendar direct you to an iphone optimized version of their sites. I've never used the iphone version of gmail but I can say that if it's anything like iphone gcal, it won't be very useful. gcal optimized for iphone doesn't allow you to enter events, and you get very limited browsing capability. no month view.
iphone's calendar app, limited as it is, is much better for navigating around dates.
and the iphone's mail app is pretty good. I have it syncing with my gmail and yahoo accounts (via imap fetch) and I like its functionality. if only iphone calendar would allow (a) syncing to gcal and (b) color-coded calendars, everything would be great.
and don't even get me started about gmail's craptastic contact management. I actually turned off nuevasync's contact sync because I don't want dozens of suggested contacts polluting my iphone and google in their infinite wisdom doesn't provide a method to filter those out.